The present invention relates generally to the control of the speed of electrical motors, and more particularly, to a system for digitally controlling the speed of a D.C. motor.
In present-day high-speed carriage drive matrix printing devices, the voltage requirements of the motor driving the carriage vary due to the number of printing elements that are firing and other load conditions. In addition, the torque on the motor changes due to the stopping and starting of the carriage as the printing mechanism completes a line of printing. Thus, the motor is subject to varying supply voltages and torque loads which affect the speed of the motor and the operation of the printing mechanism. In those printing mechanisms in which a printing carriage is operated to move the print mechanism across the printing medium, high current loads are encountered which require speed control circuits employing high-cost power transistors and other high-cost circuit elements. It is therefore a principal object of this invention to provide a speed control circuit which allows a high current D.C. motor to maintain a constant speed over a varying supply voltage and varying torque loads. It is another object of this invention to provide a low-cost speed control circuit for a high current D.C. motor which can function under the aforementioned operating conditions.